(1) Field of the Invention
The present invention relates to an image forming apparatus which includes a static latent image support and a developer support which is supported in a developing unit and retains and conveys a developer so as to produce a visual image by developing the static latent image formed on the peripheral surface of the static latent image support with the developer from the developer support.
(2) Description of the Prior Art
For electrophotographic development in a conventional, generally used image forming apparatus, dual-component development has been often used in which a non-magnetic developing sleeve incorporating a magnet roller supports a magnetic powder carrier together with toner as the developer, and supplies the toner for the development which involves a photoreceptor drum formed of .alpha.-Si, selenium, or an OPC etc.
Mono-component developing systems, which use no carrier, can be constructed of a simple and compact configuration, and are advantageous in terms of cost and maintenance. In particular, non-magnetic mono-component systems which use no magnetic toner, do not need a magnetic roller, and are advantageous in providing a developing unit which is simple in its mechanism and compact and inexpensive and still can produce clear images.
In dual-component development, the carrier which is made to stand like `spikes` develops the static latent image previously formed on the photoreceptor drum by supplying the toner carried on the carrier surface to the static latent image. The toner carried on the carrier surface has been charged beforehand with a prescribed charge by being agitated in the toner hopper by means of paddles or agitator.
Magnetic mono-component development is categorized into two classes: the non-contacting development system whereby, similarly to dual-component development, magnetic toner is conveyed by a non-magnetic developing sleeve incorporating a magnetic roller while an alternating electric field is applied across the gap having a predetermined distance between the developing sleeve and photoreceptor drum so that the toner can travel in a reciprocating manner; and the contacting development system in which the toner is made to strongly stand like `spikes` so as to bring itself in contact with the static latent image on the photoreceptor drum surface. The former needs a development bias of a d.c. voltage superimposed with an a.c. voltage and hence needs a complicated power system and a high withstand voltage configuration for the developing unit, whereas the latter may use a simple d.c. development bias power system but will produce Hi-gamma images, which are poor in gradation.
Non-magnetic mono-component development can be roughly categorized into two classes: the non-contacting development type whereby an alternating electric field is applied across the gap having a predetermined distance between the developing roller and photoreceptor drum so that the toner can travel in a reciprocating manner; and the contacting development type in which a conductive, elastic developing roller is put into contact with the photoreceptor drum. The former needs a development bias of a d.c. voltage superimposed with an a.c. voltage and hence needs a complicated power system and a high withstand voltage configuration for the developing unit, whereas the latter may use a simple d.c. development bias power source.
In any developing system, in the developing step (between the photoreceptor drum and the developing roller), if the frictions acting on the photoreceptor drum and/or the developing roller vary, jittering or banding (density unevenness and positional displacement of lines and strips along the direction perpendicular to the direction of movement of the photoreceptor drum) of 40 Hz to 100 Hz occurs as a result of backlash from the meshing of the gears and/or couplings. Such jittering or banding produces density unevenness in the image, and in the worst case, black strips and/or black and white banding unevenness perpendicular to the paper's conveyed direction may appear. In dual-component development, such a phenomenon is liable to occur due to variations in frictional resistance (load) between the carrier on the non-magnetic sleeve surface and photoreceptor drum. In mono-component developing systems, this tendency is more conspicuous in the contact type developing configuration where the elastic developing roller is in contact with the photoreceptor drum for development. In particular, in mono-component contact-development using an elastic developing roller, the developing roller is rotated at a circumferential speed different from that of the photoreceptor drum, therefore this frictional resistance may become very strong when all or almost all of the toner layer on the developing roller has been transferred to the photoreceptor drum, which may then cause image elongation.
In order to achieve an improved image quality by inhibiting the occurrence of jittering and banding, as a conventional method, Japanese Patent Application Laid-Open Hei 6 No.95,562 discloses a method in which jittering and banding are eliminated by providing an inertial body along the same axis, with an elastic damper. Japanese patent Application Laid-Open Hei 6 No.95,563 discloses another method in which a sealed oil body is abutted so as to increase the damping ratio. Japanese patent Application Laid-Open Hei 7 No.140,841 discloses a method in which a mass component is mounted via a damping component inside the photoreceptor drum so as to damp vibrations by increasing the moment of inertia.
For the above configurations, in a development system having a photoreceptor drum and a developing sleeve with magnetic brush, or a photoreceptor drum and a developing unit having a developing roller retaining toner on the surface thereof, the vibration unique to each unit vibrates the latent image and/or developed image on the photoreceptor drum thus causing jittering and/or banding in the image.
On the other hand, since the developing sleeve or developing roller produces frictional resistance against the photoreceptor drum in a manner which causes the photoreceptor drum to move in its direction of rotation, the rotation is always unstable due to the backlash of the gears and/or couplings, which results in the occurrence of jittering and/or banding in the image. In particular, since, in the magnetic brush configuration and in mono-component contact-development, the circumferential speed of the developing sleeve (developing roller) is higher than that of photoreceptor drum, the rotation of the photoreceptor drum is accelerated, which may cause image elongation.
With reference to an example of mono-component contact-development, the developing roller, for example, was configured of a homogeneous material with a uniform outside diameter across the full length including both the toner layer forming area and the toner layer non-forming area. Therefore, in contact-development in which the photoreceptor drum and the developing roller are put in pressure contact with each other with a differential relative speed, an excessive friction will arise between the photoreceptor drum and the developing roller at both ends where no toner is adhering on the developing roller, which increases and fluctuates the load torque acting on the photoreceptor drum and the developing roller. Further, this would make the rotation of the photoreceptor drum and the developing roller unstable, thus causing the problem of image unevenness with respect to the auxiliary direction (the conveyed direction of copy paper).
If the rotational irregularity is tried to be solved by making only the toner layer forming area on the developing roller come into contact with the photoreceptor drum, the degree of slipping between the developing roller and the photoreceptor drum varies depending upon the amount of development. Additionally, if an elastic developing roller is used, its outside diameter is liable to vary depending upon the change in environment, especially change in temperature, which varies the frictional resistance. So, even when high accuracy parts are used, it is impossible to perfectly eliminate the variations of the load torque.
Dual-component development also has the same defects as mono-component development since the dimensional variations of the constituent parts depending upon the environment occur in a similar manner.
In non-contacting development, variations in frictional resistance are overcome to some extent, but a vibration control device is not effective in dealing with all environments and change from aging (wear of parts) since each constituent part has a characteristic frequency different from the other.
To deal with such problems, the aforementioned method has been known in which an inertial component and dynamic damper are set in the photoreceptor drum so as to stabilize the rotation of the drum. However, this configuration needs a large inertial component in order to suppress micro variations in rotation. Alternatively, in order to stabilize the frictional resistance, a configuration using a brake has been proposed. However, both of these need a complicated mechanism for absorbing the variations in rotational speed resulting from the change of the frictional resistance and still are not very effective.
The inertial component disclosed in Japanese Patent Application Laid-Open Hei 6 Nos.95,562 and 95,563 and Hei 7 No. 140,841, is attached in the interior of the photoreceptor drum (the inertial component is integrally fixed with a dynamic damper in Japanese Patent Application Laid-Open Hei 6 No.95,562). Since the integral assembly of the photoreceptor drum and the inertial component has a considerably large mass and moment of inertia, an excessive load acts on the driving motor at activation so that the motor needs to have a large starting torque.
Naturally, there is another problem in that a long time is required until the photoreceptor drum ceases its rotation when operation stops.